The state of the art includes the structure of electric furnaces, particularly electric arc furnaces, which comprise a refractory lower shell, incorporating the hearth, above which there is the upper shell which functions as a sidewall where the cooling panels are positioned. Such furnaces also include a cover consisting of a roof which is also cooled.
In the state of the art, the sidewall of the furnace is defined by a row of these panels arranged substantially in correspondence with the outer edge of the lower shell; this situation makes possible the at least partial formation of a layer of slag, which attaches itself on the front part of the panels. However, this layer of slag is not normally enough to protect the refractory material from the thermal and chemical stresses which can be found in electric arc furnaces which are currently used.
This layer of slag has an insulating function so as to reduce the flow of heat and therefore to preserve the cooling panels from premature wear.
These solutions as are known in the state of the art, however, are not very effective since the slag attaches itself with difficulty to the inner face of the panels and cannot form a uniform and compact layer suitable to perform the function of thermal insulation.
Moreover, it is known that one of the greatest shortcomings in a furnace, as the melting cycle proceeds, is the wear and progressive erosion of the refractory material of the lower shell in the area at the level of the upper edge of the slag, that is to say, substantially at the upper circular strip of the lower shell. In this area, the combination of the temperature and the violent chemical reactions causes a high level of erosion which progressively damages the structure of the refractory material.
This forces the workers to intervene between one cycle and another so as to restore the correct conditions of efficiency of the refractory and thus obviate the risk of break-outs which are dangerous for the personnel.
Moreover, with this type of panel the heat flow directed towards the outside of the furnace is very great and a great deal of energy is lost. This is due to the great extent of the surface on which the heat exchange takes place, since the tubes forming these panels are adjacent to each other and cover the whole lateral surface of the furnace in the area where there is no refractory material.
Other shortcomings of structures including cooling panels as are known in the state of the art are that they have low resistance and are very dangerous to use.
The low resistance is caused on the one hand by the fact that only a very thin layer of slag attaches itself to the panels, and this is not sufficient to preserve the panels from the risk of breaking; and on the other hand by the great number of welds, each of which constitutes a critical point in the panels.
Moreover, the close formation of the rows of tubes and the fact that they are rigidly constrained to each other causes further thermo-mechanical stresses on the tubes, which in the long term leads to deformations and breakages caused by fatigue.
The panel structures are dangerous to use because, due to the rigid construction of the panels and the great number of welds, the tubes are subject to breakages and there is therefore a danger of water leaking out.
GB-A-2.270.146 shows an electric furnace with lateral cooling panels located above the lower shell and with cooling tubes which act on the refractory zone of the lower shell.
DE-C-4223109 shows panels with a plurality of horizontal mono-tubes arranged in two parallel rows and separated at regular intervals.
EP-A-0699885 shows a cooling system for the upper edge of the refractory part of the furnace.
This system includes U-shaped cooled tubes with the vertical tubes facing towards the bath of liquid metal.
This embodiment entails a variety of problems, on the one hand because the continuous tubes become unusable in the event of a breakage, and on the other hand because they are easily subject to perforations, since they face the bath of liquid metal.
However, if these tubes were to be protected, they would no longer have their desired effect.
In European patent application EP-A-790473, the present applicant describes a cooling device with panels for electric arc furnaces wherein, according to a first feature, there is a horizontal row of cooling tubes arranged slightly above a substantial part of the edge, or upper shoulder, of the refractory part of the lower shell and, according to another feature, there are two layers of cooling tubes, one inner and one outer, connected to each other in a rigid manner, and the pitch of the inner layer of tubes is less than that of the outer layer, in order to define spaces whereon the slag attaches itself.
This solution, although it has shown itself to be extremely satisfactory, has not completely solved all the problems and, in practice, has shown that it can be made even better, in terms of cooling efficiency, reduced energy loss, greater resistance of the tubes, lower costs, fewer risks during use, and a greater maintenance capacity.
FR-A-2.486.863 does not solve any of the above-mentioned problems, as there are no spaces whatsoever between the tubes which would allow a thick, consistent layer of slag to form.
The present applicants have designed, tested and embodied this invention in order to embody these operating improvements, and also to achieve further advantages as will be shown hereinafter.